Named Session support

[WeihanLi]

See details https://httpie.io/docs#named-sessions

Usage examples:

# Create session named user1
http --session=user1 -a user1:password pie.dev/get X-Foo:Bar

# Refer to the session by its name 
http --session=user1 pie.dev/get

# To create or reuse a different session, simple specify a different name
http --session=user2 -a user2:password pie.dev/get X-Bar:Foo

[WeihanLi]

https://github.com/httpie/httpie/blob/master/httpie/sessions.py